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Full text of "The Cambridge Natural History"

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THE relationship of Mammals to Vertebrates lying below them
in the scale, their origin in fact, is a much-debated question,
with many attempted solutions. To enter into this large
question in detail would involve a great deal of useless state-
ment of arguments founded upon misleading or upon quite in-
accurate " facts." It will perhaps be sufficient if we reflect herę
the current view most in vogue at the present, i.e. that which
would refer the Mammalia to reptiles belonging to the extinct
Permian and Triassic group of the Theromorpha (also called
Anomodontia). These have been explored lately to a very large
extent, and chiefly by Professor Seeley.1 The very fact that a
genus Tritylodon, only known by the forepart of the skull, has
been called Mammalian and Anomodont by various authors,
shows at least the difficulty of differentiating the two groups when
the material for study is imperfect. As a matter of fact
these Theromorpha are without doubt reptiles; they show, for
example, a lower jaw formed out of several distinct pieces, of
which the articular articulates with a fixed quadrate on the
skull. They possess the characteristic reptilian bones, the
"transverse/* the pre- and post-frontals, and there are various
other points of structure which leave no room for doubt as to
their truly reptilian nature. There are, however, numerous
indications of an evolution in the mammalian direction in all
parts of the skeleton, to the more important of which some
reference will be made here. It may be as well to clear the
1 A series of papers in the Phil, Trtws. for 1888-96, of which a useful abstract
by Professor Osborn was published in the Aimn&m Nctiwalist, 1898, p. 809; see
also Gambr* Ned, Hist, viii 1901, p, $03.